Various bone fixation apparatuses have been disclosed in the art, such as those described in Korean Patent Laid-open Publication No. 2000-48562 and U.S. Pat. No. 6,280,442.
As shown in FIG. 1, the bone fixation apparatus described in Korean Patent Laid-open Publication No. 2000-48562 includes a bone screw 10, a shrinkage collet 14, a receiver member 18, and a set screw 20. The bone screw 10 has a spherical head 12. The shrinkage collet 14 functions to support the head 12 of the bone screw 10. The receiver member 18 has a center bore for delimiting a tapered recess 16 in which the shrinkage collet 14 is accommodated and a U-shaped channel that communicates with the recess 16 and through which a support bar R extends. The setscrew 20 is threadedly coupled to the receiver member 18 to downwardly bias the support bar R.
The head 12 of the bone screw 10 is defined with a tool-engaging groove 22 in which a tool can be engaged. The tool engaging groove 22 is defined on a flat upper end surface 24 of the head 12, in which the upper end surface 24 is formed by truncating the head 12. A portion of the receiver member 18 which defines the U-shaped channel is formed with internal threads 26 so that the setscrew 20 can be threadedly coupled to the internal threads 26. A lower surface of the shrinkage collet 14 is formed to have a contoured depression 28 in which the head 12 is partially accommodated. In the contoured depression 28, a lower part of the shrinkage collet 14 is formed with a plurality of slots, so that a desired pressing force is applied to the head 12 of the bone screw 10.
If the setscrew 20 is tightened, the support bar R compresses the shrinkage collet 14, and the shrinkage collet 14 is squeezed within the tapered recess 16 of the receiver member 18 in such a way as to fixedly hold the bone screw 10 at a vertical or inclined position.
Meanwhile, as shown in FIG. 2, in the bone fixation apparatus described in U.S. Pat. No. 6,280,442, a series of ridges 34 are formed on a head 32 of a bone screw 30, a retainer ring 38 is fitted adjacent to a lower end of a receiver member 36 so that the head 32 can be retained by the retainer ring 38. A cap member 40 is placed on an upper part of the head 32. Above the cap member 40, a support bar R is inserted through a U-shaped channel defined in the receiver member 36 and then biased downward by a compression member 42.
In the receiver member 36, a lower part in which the cap member 40 is inserted is formed to have an inner diameter greater than that of the upper part into which the compression member 42 is threadedly coupled. Due to this fact, even in the case that the compression member 42 is unscrewed and the support bar R is removed, the cap member 40 is prevented from being released in an upward direction. When assembling the bone fixation apparatus, after the cap member 40 and the head 32 of the bone screw 30 are sequentially inserted through the lower end of the receiver member 36, the retainer ring 38 is placed around and moved upward on the bone screw 30 and then fitted into an inward annular groove defined adjacent to the lower end of the receiver member 36.
The internal threads of the receiver member 36 may be formed in a manner such that the cap member 40 is also threadedly coupled to the internal threads to be prevented from being released from the receiver member 36.
However, in the former bone fixation apparatus as described in Korean Patent Laid-open Publication No. 2000-48562, the head 12 supported in the tapered recess 16 is likely to be moved by an external factor because the supporting force is insufficient. Consequently, the head 12 cannot be reliably maintained in an initially supported state.
The latter type bone fixation apparatus as described in U.S. Pat. No. 6,280,442, while coping to some extent the problem caused in Korean Patent Laid-open Publication No. 2000-48562, suffers from defects in that it is difficult to fit the retainer ring 38 in place, and the supporting force of the bone screw is still insufficient.
When the head of the bone screw is supported by the cap member threadedly coupled to the receiver member, it is not easy to screw the cap member adjacent to the lower end of the receiver member. Further, because the biasing force of the compression member cannot be transferred to the head, assemblability is deteriorated and the supporting force of the bone screw is downgraded.